Vehicle fuel systems have long used activated carbon filled canisters to temporarily hold fuel vapors that would have been simply vented in years past. The stored vapors are desorbed from the carbon when the engine runs by using manifold vacuum to draw fresh air from ambient through the canister, and ultimately to the engine for burning. This process is generally referred to as purging, and the air drawn in referred to as purge air. In order to achieve maximum purge efficiency, purge air cannot be drawn in at a constant flow rate. Instead, various computer controls and valves are used to draw purge air at the maximum rate that can be handled by the engine under various conditions. While there is a known maximum to minimum total range of purge flow rates, the actual rate will vary continually. Another factor that can potentially affect efficiency is the fact that the purge flow may in fact be more than just air, including a significant percentage of entrained water in the form of a fine mist or droplets. While this is apparently not a well recognized problem, it is known that water in the carbon canister can adversely affect its fuel vapor adsorption capacity. In the future, even larger carbon canisters may have to be carried in vehicle locations where they will be subjected to higher water percentages in the purge inflow.
The problem of entrained water separation is recognized in the gas purification field. Louver type separators are known that pass the flow across an array of slats in such a way that it impinges upon the surface of the slats. The entrained water is stripped from the flow, collecting on the surface of the slats. It has been found that there is an optimal flow velocity, in the range of 7-10 feet per second, at which the stripped and collected water will not be picked up again by the passing flow. Therefore, it is a relatively simple matter to design the equipment so that the flow is drawn in at the optimal velocity. However, as noted above, that is not feasible in vehicle evaporative control systems, where the flow rate is variant.